Global Dimming?

It just so happens that most of the posts on this site have tried to counteract arguments from those who would sow fake “uncertainty” in the climate debate. But lest our readers feel that we are unjustifiably certain about our knowledge, let us look at a recent example of the opposite tendency: too much certainty.

A recent BBC Horizon documentary (transcript) raised the issue of ‘global dimming’ and argued that this ‘killer’ phenomena’s newly-recognised existence would lead to huge re-assessments of future global warming. As part of the hyperbole, the process of global dimming was linked very clearly to the famines in Ethiopia in the 1980s and the implication was left that worse was to come. Media reports with headlines like “Fossil Fuel Curbs May Speed Global Warming” swiftly followed. So what’s the real story?

Global dimming is indicated by measurements over land areas in many regions in the world and may therefore be a real phenomena. Though there are serious issues with the quality of some of the data (birds drinking out of uncovered evaporation pans, drift and inhomogeneities in the solar radiation measuring instruments), in the most global assessment, Beate Liepert estimated that there was globally a reduction of about 4% in solar radiation reaching the ground between 1961 and 1990. While more recent indications are that the trend may have reversed in the last decade, it could still be significant. Assuming for the sake of argument that these data are valid, what could have caused this? A change of that magnitude in the incoming solar radiation itself is not possible since satellite observations would have seen it. Thus, it must be something that is happening in the atmosphere to intercept solar radiation. There are only a few possibilities: clouds, water vapour or aerosols.

First of all it is important to note that even pure greenhouse gas forcing will lead to a slight decrease in surface solar radiation (due to the concurrent increased humidity) and potential cloud feedbacks. Cloud cover and thickness are both like to vary as a function of climate change.

Contrails (those wispy trails left behind high flying jets) have increased over the period and may be important. But estimates of their global effect, even making very generous assumptions about their spread are small (Minnis et al, 2004). Aerosols are also known to have increased over this time, and so they are a natural candidate. However, simulations using the relatively straightforward ‘direct effect’ of aerosols (the increase in albedo of the planet due to the particle brightness) do not match the inferred changes. The final candidates are numerous interactions of aerosols with clouds, the so-called ‘indirect effects’.

There are an ever increasing number of these ‘indirect effects’, but the two most discussed are the aerosol/cloud opacity interaction (more aerosols provide more sites for water to condense in clouds, thus cloud droplets are smaller and clouds become more opaque), and the cloud lifetime effect (smaller droplets make it more difficult to make drops big enough to rain, and so clouds live longer). Estimates of the importance of such effects vary widely, and while they are thought to be significant, the uncertainty associated with them is very large. These effects are nevertheless a necessary part of the suite of human-related forcings that are being assessed in order to understand the climate of the 20th Century.